Gas masks and protective clothing are generally employed to protect persons working in an atmosphere containing toxic gases, vapors, fumes, smoke and the like resulting from industrial disasters, military attack, etc. In some situations it is necessary to provide protective shelters, e.g. compartments, vehicles and vans, wherein personnel can be free from the restraints of masks and protective clothing to assure completion of the mission during the emergency. Such shelters must be provided with an air conditioner unit and a filter unit containing a filter for removing toxic gases and a filter for removing particulate matter. The gas-particulate filter unit contains a fan or blower to force the ambient air through the filters, which remove the toxic gases and particulate matter. The purified air from the filter unit then flows through an air duct to the fresh air inlet of the air conditioner unit which cools and circulates the pruified air into the shelter.
When the gas-particulate filter unit is thus connected to the fresh air inlet of the air conditioner, all make-up air to the air conditioner must pass through the filter unit. However, the air conditioner unit cannot draw adequate make-up air through the filter unit when the filter unit fan is not operating. Consequently, a bypass valve is required in the air duct between the filter unit and the air conditioner unit, which would be open to the atmosphere when the filter unit fan is not in operation and thus provide make-up air to the air conditioner unit. A manually operated bypass valve is unsatisfactory, since it leaves room for human error, while an electrically operated valve is subject to electrical problems.